Aqueous fluoropolymer dispersions are typically manufactured in an aqueous dispersion polymerization employing anion fluorosurfactants as a polymerization aid, the fluorosurfactants functioning as a non-telogenic dispersing agent. For example, an early description of this use of anionic fluorosurfactants is found in U.S. Pat. No. 2,559,752 (Berry). After polymerization, the dispersions are usually subjected to a concentration step to increase the fluoropolymer solids concentration in the dispersion. In one type of dispersion concentration operation, the dispersion is concentrated with the aid of a nonionic surfactant as taught in Marks et al., U.S. Pat. No. 3,037,953, and in Holmes, U.S. Pat. No. 3,704,272 to raise the solids from nominally 35 wt % in the raw dispersion to about 60 wt % in the concentrated dispersion. Miura et al., U.S. Pat. No. 6,153,688 discloses a similar process. Anionic fluorosurfactants and nonionic surfactants are usually present in the concentrated dispersions.
Because of environmental concerns and because anionic fluorosurfactants are expensive, processes have been developed for the removal of anionic fluorosurfactants from aqueous fluoropolymer dispersions. One method for removal of anionic fluorosurfactants from fluoropolymer dispersions is disclosed in U.S. Pat. No. 4,369,266 and includes the addition of a stabilizing surfactant followed by concentration by ultrafiltration. This patent teaches that a high proportion of the fluorosurfactant can be removed via the aqueous permeate. It is also known to remove anionic fluorosurfactant by adsorption onto an ion exchange resin as taught in U.S. Pat. No. 3,882,153 (Seki et al) and U.S. Pat. No. 4,282,162 (Kuhls). Kuhls teaches recovery of fluorinated emulsifiers dissolved in the aqueous phase after coagulation of the polymer from the dispersion or in aqueous polymer dispersions to be concentrated. In anion exchange processes, the anionic fluorosurfactant is removed by the anion exchange resin from a stabilized dispersion containing nonionic surfactant.
In concentrated aqueous fluoropolymer dispersions which have reduced levels of anionic fluorosurfactant, the viscosity levels can be higher than in dispersions containing fluorosurfactant and can be unacceptably high for some end uses. Certain types of fluoropolymer dispersion, particularly high molecular weight polytetrafluoroethylene dispersions, show an increase to an unusually high viscosity when the anionic fluorosurfactant content is significantly reduced. Viscosity can rise to a level of several hundred centipoise (cP), well above the normal 20-30 cP which is advantageous for coating and impregnating compositions and to make cast films. US 2004/0186219 A1 and US 2003/0171736 A1 (Dadelas et al.) teach the addition of a non-fluorinated anionic surfactant, e.g., sodium lauryl sulfate, sodium dodecylbenzyl sulphonate and secondary alkyl sulphonate sodium salt, to reduce viscosity. However, such non-fluorinated anionic surfactants introduce ionic species into the dispersion which are not volatile under the conditions used for drying and sintering the fluoropolymer in coating or film casting operations. The presence of residues from such species can produce objectionable color in films and coatings and may give rise to long term stability problems.